1. Field of the Invention
Broadly speaking, this invention relates to television systems and the like. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for reducing the bandwidth of a video signal for transmission over a communication facility of limited bandwidth.
2. Description of the Prior Art
One of the major factors to be considered in the design of any switched video telephone system is the overall cost to the subscriber. Because broad-band video channels are expensive it has generally been agreed, both here and in Europe, that transmission facilities having a bandwidth of about 1 MHz represent a reasonable compromise between technical and economic considerations.
Once system bandwidth is determined, attention becomes focused on the standards to be employed by the camera and receiver at the subscriber's location. Logically, one would expect the camera and receiver to have bandwidth requirements approximately the bandwidth of the transmission facility used. Bell System PICTUREPHONE service, for example, employs a 267 line, 30 frames per second scanning standard and produces a video signal of approximately 1.2 MHz bandwidth, which is close to the 1 MHz bandwidth of PICTUREPHONE transmission channels.
Because PICTUREPHONE scanning standards differ from commercial scanning standards, each PICTUREPHONE camera and receiver must be specially manufactured, which adds to the overall cost of providing PICTUREPHONE service. To obviate this, it has been proposed that standard, off-the-shelf television cameras and receivers be modified for use with the PICTUREPHONE system. This would permit the economies of scale which are available to commercial equipment manufacturers to be passed on to the subscriber, for example, by way of reduced rates. Unfortunately, the high definition video signal from a commercial television camera has a bandwidth of approximately 4 MHz and if this broad-band video signal is transmitted over a 1 MHz transmission channel without special processing, the resulting picture, as received at a distant location, is psychologically objectionable to most viewers. This is due to the fact that the vertical resolution of the received picture, which is determined primarily by the number of active scanning lines, will be approximately four times greater than the horizontal resolution, which is determined primarily by the bandwidth of the transmission facility. Thus, some means for approximately equalizing the subjective perception of both horizontal and vertical resolution by the viewer is desirable.
An article by van Buul and van de Polder in Philips Research Reports, Volume 28, August 1973, pages 377 - 390, describes a technique for processing a broadband, 625 line, 25 frames/second, video signal for transmission over a 1 MHz channel for ultimate display on a 313 line, 25 frames/second video telephone.
The technique described by van Buul and van de Polder is straightforward in its approach. Briefly, even-numbered scanning lines are discarded and odd numbered lines are alternately fed to a pair of analogue shift registers which are shifted at a rate corresponding to the horizontal frequency of the incoming 625 line picture. Subsequently, the shift registers are read out at a rate very nearly corresponding to the horizontal frequency of the 313 line picture. This has the effect of stretching every alternate line in the original 625 line picture. The line time stretching, combined with the use of only alternate lines, reduces the bandwidth of the video signal from 5 MHz down to about 1 MHz, thus matching the 1 MHz bandwidth of the transmission facility.
Unfortunately, this relatively unsophisticated approach to bandwidth reduction results in a certain degree of picture degradation. For example, because the original 625 line picture is interlaced 2:1, the scanning lines in alternate fields of the reduced bandwidth, 313 line picture are displaced upwardly from their desired position by an amount which is equal to the distance between adjacent lines in the 625 line picture. This causes any oblique lines which may be present in the scanned scene to take on a serrated appearance and, in addition, generates moire patterns in higher spatial frequencies in the vertical direction. These effects can be minimized by the use of more complicated circuitry including delay lines, but the circuitry then becomes considerably more expensive to manufacture. A somewhat similar scheme is disclosed in U.S. Pat. No. 3,806,644, which issued on Apr. 23, 1974 to P. N. Browne et al and which is assigned to the assignee of the instant invention. Unlike van Buul's and van de Polder's technique, however, Browne et al disclose apparatus for converting a 267 line, 30 frames/second PICTUREPHONE signal for display on a commercial receiver originally intended to display a standard 525 line, 30 frames/second picture. However, the circuitry disclosed by Browne is generally similar to that employed by van Buul and van de Polder.
The problem, then, is to convert the output of a television camera nominally designed to produce a 525 line, 30 frames/second, 2:1 interlaced picture so that it is compatible with a 1 MHz PICTUREPHONE transmission channel and then to display the resultant picture either on a standard PICTUREPHONE Video Telephone (PVT) receiver or, after further processing, on a standard commercial television receiver, all without substantial degradation of the picture with respect to established PICTUREPHONE standards.